In a known high pressure fuel pump, a rotor is provided with one or more bores, pumping plungers being reciprocable within the bores. The outer end of each pumping plunger has a shoe and roller arrangement associated therewith, the rollers being engageable with the cam surface of a cam ring. As the rollers ride over cam lobes forming part of the cam surface during rotary motion of the rotor with respect to the cam ring, the plungers are pushed inwardly into the respective bores, pressurizing and displacing fuel from the bores to permit fuel to be supplied under pressure to the cylinders of an associated engine.
It will be appreciated that the timing of fuel delivery by the pump is dependent upon the position of the cam lobes, and hence upon the angular position of the cam ring. The timing of fuel delivery can therefore be advanced or retarded by moving the cam ring. One way of moving the cam ring is to provide the cam ring with an outwardly extending peg which is received within a recess or opening provided in an advance piston. The piston is slidable within a bore, and is spring biased towards one end of the bore. Fuel under pressure is supplied to the bore, the fuel pressure acting on a surface of the piston to apply a force to the piston acting against the spring. In such an arrangement, variations in the fuel pressure applied to the bore result in the advance piston occupying different axial positions, the engagement between the piston and the peg transmitting axial movement of the piston to the cam ring, adjusting the angular position of the cam ring.
The fuel supplied to the advance piston is conveniently drawn from a low pressure feed pump which is used to supply fuel to the high pressure pump. The output pressure of the feed pump is conveniently controlled so as to be related to engine speed, and in such an arrangement, the timing of fuel delivery by the high pressure pump is related to engine speed. It is known to provide a valve arrangement between the feed pump and the advance piston whereby the fuel pressure applied to the advance piston, and hence the timing of fuel delivery, can be controlled independently of engine speed.
It is advantageous to minimize the quantity of fuel drawn from the feed pump for purposes other than supplying the high pressure pump with fuel in order to reduce the maximum capacity of the feed pump.